This whole issue confuses me, I think mainly because I am sure that if Cindy found the diary in the beginning, we wouldn't be looking at it now, maybe it just wasn't there at that time.
We know Cindy has no real moral compass and would have destroyed such an incriminating piece of evidence, just as she tried to throw LE off by giving them a family hairbrush, rather than Caylee's, and has consistently lied to save her daughter having to pay for her crime. She would go to any lengths to save Casey.
It would seem also, if one had just caused the death of one's own child, then to proceed to write such self incriminating diary entries would be really counter intuitive to say the least. Having done that, then to leave the diary hanging around for anyone to find, but removing just some of the pages, while leaving what is essentially a confession, is not only highly unlikely but foolhardy.
Perhaps the diary was written after the fact and after the initial searches of the Anthony home. I have wondered if Casey's hatred of her mother caused her to write this little malignant piece to further wound her mother while on the visits to her lawyers offices or when she was under house arrest.
Anyway, the diary was found by Cindy, as was intended, and the date had been added to confuse anyone else who found it, with the exception of Cindy, who I believe may have known all along it was created, not while KC was with TL but after Cindy called LE.
After LE's surprise search of the home, when the Anthony's were dining on crab puffs at some swanky hotel, Cindy had missed the opportunity to get rid of the diary and LE photographed it and added it to their evidence. I believe Cindy gave it to BC to make it less accessible to LE with the hope that they just may forget all about it or could somehow prevent it being used at trial.
The message to Cindy is loud and clear, Casey hates her mother and wants her to know she is responsible, and she did it because Cindy dared love Calylee as much, if not more than her daughter. If it means she has to cause Cindy life long agony by being incarcerated, she is ok with that too...Just more grist for the mill, It seems everything Casey does is aimed directly at her mother. It's payback ..............JMO